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arXiv2012

Molecular simulation of surface reorganization and wetting in crystalline cellulose I and II

Cellulose is one of the most versatile substances in the world. Its immense variety of applications was in recent years complemented by interesting nanotechnological applications. The fabrication of complex cellulose-based materials asks for thorough understanding of the surface structure and its interactions with adsorbates. In this study we investigate several surface model systems of nanotechnological interest, which are obtained by reorganization of the cellulose-vacuum or cellulose-water interfaces of slabs of crystalline cellulose. To do this, we constructed supercells from crystallographic data, and then optimized the interface structures. From the bulk and surface systems we calculated structural properties such as unit cell parameters, dihedral conformation distributions, density profiles and hydrogen bonding. The results suggest that no overall geometrical restructuring occurs at the interface. However, the hydrogen bond network is strongly reconstructed, as is inferred from the dihedral conformations and hydrogen bond occurrences, although only within the first few layers. This holds for low index close packed structures as well as for high index loosely packed surfaces. Replacing the vacuum by ambient pressure water molecules we find less rearrangements of the cellulose surface, because the water allows formation of hydrogen bonds similar to those in the bulk phase. The water near the cellulose surface shows, however, strong structural changes. We observe reduced mobility of the water molecules, which corresponds to a cooling of water by about 30 degrees, in a slab that is about 10 Å thick. Although structuring and adsorption is observed on all surfaces, no actual penetration of water into the cellulose structure could be observed. This suggests that pure water is not sufficient to produce cellulose swelling at mesoscopic timescales.

cond-mat.mtrl-sci↗

arXiv2012

Defects and glassy dynamics in solid He-4: Perspectives and current status

We review the anomalous behavior of solid He-4 at low temperatures with particular attention to the role of structural defects present in solid. The discussion centers around the possible role of two level systems and structural glassy components for inducing the observed anomalies. We propose that the origin of glassy behavior is due to the dynamics of defects like dislocations formed in He-4. Within the developed framework of glassy components in a solid, we give a summary of the results and predictions for the effects that cover the mechanical, thermodynamic, viscoelastic, and electro-elastic contributions of the glassy response of solid He-4. Our proposed glass model for solid He-4 has several implications: (1) The anomalous properties of He-4 can be accounted for by allowing defects to freeze out at lowest temperatures. The dynamics of solid He-4 is governed by glasslike (glassy) relaxation processes and the distribution of relaxation times varies significantly between different torsional oscillator, shear modulus, and dielectric function experiments. (2) Any defect freeze-out will be accompanied by thermodynamic signatures consistent with entropy contributions from defects. It follows that such entropy contribution is much smaller than the required superfluid fraction, yet it is sufficient to account for excess entropy at lowest temperatures. (3) We predict a Cole-Cole type relation between the real and imaginary part of the response functions for rotational and planar shear that is occurring due to the dynamics of defects. Similar results apply for other response functions. (4) Using the framework of glassy dynamics, we predict low-frequency yet to be measured electro-elastic features in defect rich He-4 crystals. These predictions allow one to directly test the ideas and very presence of glassy contributions in He-4.

cond-mat.mtrl-sci↗

arXiv2013

Low-temperature specific heat of graphite and CeSb2: Validation of a quasi-adiabatic continuous method

We present the application of a fast quasi-adiabatic continuous method to the measurement of specific heat at 4He temperatures, which can be used for the study of a wide range of materials. The technique can be performed in the same configuration used for the relaxation method, as the typical time constants between calorimetric cell and thermal sink at 4.2 K are chosen to be of the order of tau~30 s. The accuracy in the absolute values have been tested by comparing them to relaxation-method results obtained in the same samples (performed in situ using the same set-up), with a deviation between the absolute values < 3% in the whole temperature range. This new version of the continuous calorimetric method at low temperatures allows us to completely characterize and measure a sample within a few hours with a high density of data points, whereas when employing other methods we typically need a few days. An exhaustive study has been performed for reproducibility to be tested. In the present work, we have applied this method to two different substances: CeSb2, which exhibits three magnetic transitions at 15.5 K, 11.7 K and 9.5 K, and graphite, both highly-oriented pyrolytic graphite (HOPG) and natural crystals. Our results on these graphites are discussed in comparison with previous published data on different kinds of graphite samples.

cond-mat.mtrl-sci↗

arXiv2013

Modification of emission properties of ZnO layers due to plasmonic near-field coupling to Ag nanoislands

A simple fabrication method of Ag nanoislands on ZnO films is presented. Continuous wave and time-resolved photoluminescence and transmission are employed to investigate modifications of visible and UV emissions of ZnO brought about by coupling to localized surface plasmons residing on Ag nanoislands. The size of the nanoislands, determining their absorption and scattering efficiencies, is found to be an important factor governing plasmonic modification of optical response of ZnO films. The presence of the Ag nanoislands of appropriate dimensions causes a strong (threefold) increase in emission intensity and up to 1.5 times faster recombination. The experimental results are successfully described by model calculations within the Mie theory.

cond-mat.mtrl-sci↗

arXiv2009

Microstructure-dependent local strain behavior in polycrystals through in situ scanning electron microscope tensile experiments

Digital image correlation of laser-ablated platinum nanoparticles on the surface of a polycrystalline metal (nickel-based superalloy Rene 88DT) was used to obtain the local strain behavior from an in situ scanning electron microscope tensile experiment at room temperature. By fusing this information with crystallographic orientations from EBSD, a subsequent analysis shows that the average maximum shear strain tends to increase with increasing Schmid factor. Additionally, the range of the extreme values for the maximum shear strain also increases closer to the grain boundary, signifying that grain boundaries and triple junctions accumulate plasticity at strains just beyond yield in polycrystalline Rene 88DT. In situ experiments illuminating microstructure-property relationships of this ilk may be important for understanding damage nucleation in polycrystalline metals at high temperatures.

cond-mat.mtrl-sci↗

arXiv2013

Characterizing the local primary dendrite arm spacing in directionally-solidified dendritic microstructures

Characterizing the spacing of primary dendrite arms in directionally-solidified microstructures is an important step for developing process-structure-property relationships by enabling the quantification of (i) the influence of processing on microstructure and (ii) the influence of microstructure on properties. In this work, we utilized a new Voronoi-based approach for spatial point pattern analysis that was applied to an experimental dendritic microstructure. This technique utilizes a Voronoi tessellation of space surrounding the dendrite cores to determine nearest neighbors and the local primary dendrite arm spacing. In addition, we compared this technique to a recent distance-based technique and a modification to this using Voronoi tesselations. Moreover, a convex hull-based technique was used to include edge effects for such techniques, which can be important for thin specimens. These methods were used to quantify the distribution of local primary dendrite arm spacings, their spatial distribution, and their correlation with interdendritic eutectic particles for an experimental directionally-solidified Ni-based superalloy micrograph. This can be an important step for correlating with both processing and properties in directionally-solidified dendritic microstructures.

cond-mat.mtrl-sci↗

arXiv2021

Sustainable low-cost method for production of High entropy alloys from alloy scraps

In this communication,we propose a sustainable way to produce high entropy alloys from alloy scraps called Alloy mixing.We successfully demonstrate this method using a near equimolar CrCuFeMnNi HEA. Alloy scraps (304L stainless steel, Nichrome 80 and electrical wire grade Copper) obtained from various sources were melted together using vacuum arc melting along with minor additions of Mn and Cr to achieve the equiatomic composition.The alloy was characterized using X-ray Diffraction (XRD) and Scanning Electron Microscopy, which confirmed that the alloy produced through Alloy mixing exhibits a microstructure similar to that of the alloy with the same composition produced through conventional melting of pure elements

cond-mat.mtrl-sci↗

arXiv2013

Theoretical and numerical study of lamellar eutectoid growth influenced by volume diffusion

We investigate the lamellar growth of pearlite at the expense of austenite during the eutectoid transformation in steel. To begin with, we extend the Jackson-Hunt-type calculation (previously used to analyze eutectic transformation) to eutectoid transformation by accounting for diffusion in all the phases. Our principal finding is that the growth rates in presence of diffusion in all the phases is different as compared to the case when diffusion in growing phases is absent. The difference in the dynamics is described by a factor 'ρ' which comprises of the ratio of the diffusivities of the bulk and the growing phases, along with the ratios of the slopes of the phase co-existence lines. Thereafter, we perform phase-field simulations, the results of which are in agreement with analytical predictions. The phase-field simulations also reveal that diffusion in austenite as well as ferrite leads to the formation of tapered cementite along with an overall increase in the transformation kinetics as compared to diffusion in austenite (only). Finally, it is worth noting that the aim of present work is not to consider the pearlitic transformation in totality, rather it is to isolate and thereby investigate the influence of diffusivity in the growing phases on the front velocity.

cond-mat.mtrl-sci↗

arXiv2013

Phase-decomposition-related short-range ordering in a Fe-Cr alloy

A redistribution of Cr atoms related to a phase decomposition (PD) caused by an isothermal annealing at 415 C in a 15.15 at%Cr Fe-Cr alloy was studied in an ex-situ way by the conversion electrons Mössbauer spectroscopy. Analysis of the spectra in terms of a two-shell model enabled determination of probabilities of 17 different atomic configurations and average numbers of Cr atoms within the first (1NN) and the second (2NN) neighbor shells versus annealing time, separately. The annealing-time evolution of these numbers, expressed in terms of the Cowley-Warren short-range order (SRO) parameters, was shown to follow the Johnson-Mehl-Avrami-Kolgomorov equation. The SRO-parameter averaged over the 1NN-2NN shells was revealed to be linearly correlated with the average hyperfine field. Signatures in favor of the nucleation and growth mechanisms responsible for PD are discussed, too.

cond-mat.mtrl-sci↗

arXiv2013

Structural, electrical and magnetic studies of Co:SnO2 and (Co,Mo):SnO2 films prepared by pulsed laser deposition

Here we report on the structural, optical, electrical and magnetic properties of Co-doped and (Co,Mo)-codoped SnO2 thin films deposited on r-cut sapphire substrates by pulsed laser deposition. Substrate temperature during deposition was kept at 500 C. X-ray diffraction analysis showed that the undoped and doped films are crystalline with predominant orientation along the [101] direction regardless of the doping concentration and doping element. Optical studies revealed that the presence of Mo reverts the blue shift trend observed for the Co-doped films. For the Co and Mo doping concentrations studied, the incorporation of Mo did not contribute to increase the conductivity of the films or to enhance the ferromagnetic order of the Co-doped films.

cond-mat.mtrl-sci↗

arXiv2013

Phase growth control in low temperature PLD Co:TiO2 films by pressure

This paper reports on the structural and optical properties of Co-doped TiO2 thin films grown onto (0001) Al2O3 substrates by non-reactive pulsed laser deposition (PLD) using argon as buffer gas. It is shown that by keeping constant the substrate temperature at as low as 310 C and varying only the background gas pressure between 7 Pa and 70 Pa, it is possible to grow either epitaxial rutile or pure anatase thin films, as well as films with a mixture of both polymorphs. The optical band gaps of the films are red shifted in comparison to the values usually reported for undoped TiO2, which is consistent with n-type doping of the TiO2 matrix. Such band gap red shift brings the absorption edge of the Co-doped TiO2 films into the visible region, which might favour their photocatalytic activity. Furthermore, the band gap red shift depends on the films phase composition, increasing with the increase of the Urbach energy for increasing rutile content.

cond-mat.mtrl-sci↗

arXiv2013

The influence of residual oxidizing impurities on the synthesis of graphene by atmospheric pressure chemical vapor deposition

The growth of graphene on copper by atmospheric pressure chemical vapor deposition in a system free of pumping equipment is investigated. The emphasis is put on the necessity of hydrogen presence during graphene synthesis and cooling. In the absence of hydrogen during the growth step or cooling at slow rate, weak carbon coverage, consisting mostly of oxidized and amorphous carbon, is obtained on the copper catalyst. The oxidation originates from the inevitable occurrence of residual oxidizing impurities in the reactor's atmosphere. Graphene with appreciable coverage can be grown within the vacuum-free furnace only upon admitting hydrogen during the growth step. After formation, it is preserved from the destructive effect of residual oxidizing contaminants once exposure at high temperature is minimized by fast cooling or hydrogen flow. Under these conditions, micrometer-sized hexagon-shaped graphene domains of high structural quality are achieved.

cond-mat.mtrl-sci↗

arXiv2013

AFLOW: An automatic framework for high-throughput materials discovery

Recent advances in computational materials science present novel opportunities for structure discovery and optimization, including uncovering of unsuspected compounds and metastable structures, electronic structure, surface, and nano-particle properties. The practical realization of these opportunities requires systematic generation and classification of the relevant computational data by high-throughput methods. In this paper we present Aflow (Automatic Flow), a software framework for high-throughput calculation of crystal structure properties of alloys, intermetallics and inorganic compounds. The Aflow software is available for the scientific community on the website of the materials research consortium, aflowlib.org. Its geometric and electronic structure analysis and manipulation tools are additionally available for online operation at the same website. The combination of automatic methods and user online interfaces provide a powerful tool for efficient quantum computational materials discovery and characterization.

cond-mat.mtrl-sci↗

arXiv2012

Density-functional study of LixMoS2 intercalates (0<=x<=1)

The stability of Lithium intercalated 2H- and 1T allotropes of Molybdenum disulfide (LixMoS2) is studied within a density-functional theory framework as function of the Li content (x) and the intercalation sites. Octahedral coordination of Li interstitials in the van der Waals gap is found as the most favorite for both allotropes. The critical content of Lithium, required for the initialization of a 2H->1T phase transition is estimated to x ~ 0.4. For smaller Li contents the hexagonal 2H crystal structure is not changed, while 1T-LixMoS2 compounds adopt a monoclinic lattice. All allotropic forms of LixMoS2 - excluding the monoclinic Li1.0MoS2 structure - show metallic-like character. The monoclinic Li1.0MoS2 is a semiconductor with a band gap of 1.1 eV. Finally, the influence of Li intercalation on the stability of multiwalled MoS2 nanotubes is discussed within a phenomenological model.

cond-mat.mtrl-sci↗

arXiv2013

Hydrogen accommodation in Zr second phase particles: Implications for H pick-up and hydriding of Zircaloy-2 and Zircaloy-4

Ab-initio computer simulations have been used to predict the energies associated with the accommodation of H atoms at interstitial sites in α, β-Zr and Zr.M intermetallics formed with common alloying additions (M = Cr, Fe, Ni). Intermetallics that relate to the Zr2(Ni,Fe) second phase particles (SPPs) found in Zircaloy-2 exhibit favourable solution enthalpies for H. The intermetallic phases that relate to the Zr(Cr,Fe)2 SPPs, found predominantly in Zircaloy-4, do not offer favourable sites for interstitial H. It is proposed that Zr(Cr,Fe)2 particles may act as bridges for the migration of H through the oxide layer, whilst the Zr2(Ni,Fe)-type particles will trap the migrating H until these are dissolved or fully oxidised.

cond-mat.mtrl-sci↗

arXiv2013

Enhancement of DFT-calculations at Petascale: Nuclear Magnetic Resonance, Hybrid Density Functional Theory and Car-Parrinello calculations

One of the most promising techniques used for studying the electronic properties of materials is based on Density Functional Theory (DFT) approach and its extensions. DFT has been widely applied in traditional solid state physics problems where periodicity and symmetry play a crucial role in reducing the computational workload. With growing compute power capability and the development of improved DFT methods, the range of potential applications is now including other scientific areas such as Chemistry and Biology. However, cross disciplinary combinations of traditional Solid-State Physics, Chemistry and Biology drastically improve the system complexity while reducing the degree of periodicity and symmetry. Large simulation cells containing of hundreds or even thousands of atoms are needed to model these kind of physical systems. The treatment of those systems still remains a computational challenge even with modern supercomputers. In this paper we describe our work to improve the scalability of Quantum ESPRESSO \cite{Quantum-Espresso} for treating very large cells and huge numbers of electrons. To this end we have introduced an extra level of parallelism, over \emph{\emph{electronic bands}}, in three kernels for solving computationally expensive problems: the Sternheimer equation solver (Nuclear Magnetic Resonance, package QE-GIPAW), the Fock operator builder (electronic ground-state, package PWscf) and most of the Car-Parrinello routines (Car-Parrinello dynamics, package CP). Final benchmarks show our success in computing the Nuclear Magnetic Response (NMR) chemical shift of a large biological assembly, the electronic structure of defected amorphous silica with hybrid exchange-correlation functionals and the equilibrium atomic structure of height Porphyrins anchored to a Carbon Nanotube, on many thousands of CPU cores.

cond-mat.mtrl-sci↗

arXiv2013

Graphics Processing Unit acceleration of the Random Phase Approximation in the projector augmented wave method

The Random Phase Approximation (RPA) for correlation energy in the grid-based projector augmented wave (gpaw) code is accelerated by porting to the Graphics Processing Unit (GPU) architecture. The acceleration is achieved by grouping independent vectors/matrices and transforming the implementation from being memory bound to being computation/latency bound. With this approach, both the CPU and GPU implementations have been enhanced. We tested the GPU implementation on a few representative systems: molecules (O2), bulk solids (Li2O and MoO3) and molecules adsorbed on metal surfaces (N2/Ru(0001) and CO/Ni(111)). Improvements from 10+ to 40+ have been achieved (8-GPUs versus 8-CPUs). A realistic RPA calculation for CO/Ni(111) surface can be finished in 5.5 h using 8 GPUs. It is thus promising to employ non-self-consistent RPA for routine surface chemistry simulations.

physics.comp-ph↗

arXiv2013

Raman spectroscopy investigation of the H content of heated hard amorphous carbon layers

We revisit here how Raman spectroscopy can be used to estimate the H content in hard hydrogenated amorphous carbon layers. The H content was varied from 2 at.% to 30 at.%, using heat treatments of a a-C:H, from room temperature to 1300 K and was determined independently using ion beam analysis. We examine the correlation of various Raman parameters and the consistency of their thermal evolution with thermo-desorption results. We identify a weak band at 860 cm-1 attributed to H bonded to C(sp2). We show that the HD/HG parameter (Height ratio between the D and G bands) is quasi-linear in the full range of H content and can thus be used to estimate the H content. Conversely, we show that the m/HG parameter (ratio between the photoluminescence background, m, and the height of the G band), often used to estimate the H content, should be used with care, first because it is sensitive to various photoluminescence quenching processes and second because it is not sensitive to H bonded to C(sp2).

cond-mat.mtrl-sci↗